What to check on a Tank Cleaning System (COW/Butterworth).
Tank cleaning systems on tankers use fixed, remotely or hydraulically driven rotating nozzle machines mounted through the tank top to wash internal surfaces without anyone entering the tank. Crude oil washing (COW) uses heated crude cargo, drawn from the ship's own cargo system during discharge, as the washing medium; the oil dissolves wax and paraffin deposits that plain water cannot touch, and MARPOL made COW mandatory on eligible new crude carriers because it recovers cargo residue instead of discharging it. Butterworth machines, named after the original manufacturer but now a…
What makes this type
Tank cleaning systems on tankers use fixed, remotely or hydraulically driven rotating nozzle machines mounted through the tank top to wash internal surfaces without anyone entering the tank. Crude oil washing (COW) uses heated crude cargo, drawn from the ship's own cargo system during discharge, as the washing medium; the oil dissolves wax and paraffin deposits that plain water cannot touch, and MARPOL made COW mandatory on eligible new crude carriers because it recovers cargo residue instead of discharging it. Butterworth machines, named after the original manufacturer but now a generic term, wash with seawater or recycled slop and are used for product change, gas-freeing preparation, or general tank cleaning where crude washing does not apply. Both use the same basic mechanism: a programmed or continuously rotating nozzle head delivering high-pressure jets in a pattern that covers the full tank internal surface over a wash cycle.
Main components
Deck-mounted machine and hydraulic or mechanical drive
Sits on a deck stool above a tank cleaning opening, driven by a hydraulic motor or, on older mechanical types, by water turbine action from the jet itself.
Rotating nozzle head
Produces a two-plane rotation pattern so the jet sweeps the full internal surface over a programmed cycle time, typically covering the tank in one or more fixed-height positions or a continuous programmable pattern.
Supply piping and heating arrangement
Delivers washing medium at the required pressure and, for COW, at elevated temperature drawn from the cargo heating system.
Portable vs fixed machines
Fixed machines are permanently piped in; portable machines are rigged through deck openings for spot cleaning where fixed coverage is insufficient.
Control station
Local or remote control of wash sequence, pressure and duration, interlocked on COW systems with inert gas and oxygen content monitoring in the tank.
Selection / Sizing
- Tank geometry and internal structure (stringers, swash bulkheads) that determine machine placement and number of positions needed for full coverage.
- Required wash pressure and flow rate, generally higher for COW to achieve adequate oil-jet cleaning energy.
- Cargo type range: COW is only applicable to crude oil cargoes; product and chemical tankers rely on Butterworth-type washing exclusively.
- Compatibility with the ship's tank cleaning heater and cargo pump capacity, since COW draws directly from the cargo system.
Regulations / Class
MARPOL Annex I requires COW systems and a COW operations and equipment manual on eligible new crude oil tankers, with the wash pattern and machine coverage subject to approval as part of that manual. Tank atmosphere must be monitored for oxygen content during COW operations per the manual's procedures, since washing crude in an enriched atmosphere is a well-known explosion risk. Classification societies survey the fixed piping, machine mountings and heating arrangement as part of the cargo system survey.
Typical faults
| Fault | Consequence |
|---|---|
| Nozzle bearing wear causing erratic or stalled rotation | Uneven coverage leaves wax and sludge patches, increasing manual cleaning burden and gas-freeing time |
| Hydraulic drive seal failure | Loss of drive pressure, machine stops mid-cycle, oil contamination of the hydraulic system |
| Deck stool or gland leakage | Vapour or product leak at deck level, a fire and pollution risk during operation |
| Heating coil scaling reducing wash medium temperature | Reduced COW cleaning effectiveness, more residue left for manual mucking |
| Program controller fault on programmable machines | Machine defaults to a fixed spray position, missing sections of the tank surface |
What to look for in a supplier
- Documented coverage pattern calculations for the specific tank geometry, not a generic machine placement drawing.
- Spares and service support recognising yard-standard interchangeability requirements on many fleets.
- Materials compatible with the cargo range if the ship also carries chemicals or specialty products through the same tanks.
- Track record with the class society's COW manual approval process, since a poorly documented submission delays newbuilding delivery.
A machine that sounds different on start-up, even briefly, is usually a bearing going before it is a drive fault; log it and inspect at the next tank entry rather than waiting for it to stall mid-wash.